Ions such as silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium ions have been known for a long time as metal ions which exhibit antifungal, antibacterial and antialgal activities (hereinafter referred to as "antimicrobial metal ions"), and particularly silver ion has been widely used in the form of an aqueous silver nitrate solution as bactericides or disinfectants. However, the above-mentioned metal ions having antifungal, antibacterial and antialgal activities are, in many cases, toxic for human bodies and have various restrictions in methods of use, storage and disposal, and their uses are limited.
Recently, it has become clear that application of a slight amount of an antimicrobial metal to subjects is enough to exhibit antifungal, antibacterial and antialgal activities, and organic microbicides or antimicrobial compounds comprising antimicrobial metals supported on ion exchange resins or chelate resins, and inorganic microbicides or antimicrobial compounds comprising antimicrobial metals supported on clay minerals, inorganic ion exchangers or porous materials have been proposed as microbicides having antifungal, antibacterial and antialgal activities.
Among the above microbicides, the inorganic microbicides are generally higher in safety, have longer period of time showing antimicrobial effect and are superior in heat resistance as compared with the organic microbicides.
As one of the inorganic microbicides, there are microbicides prepared by replacing an alkali metal ion such as sodium ion in clay minerals such as montmorillonite and zeolite with silver ion, but the skeleton structure of the clay minerals per se is inferior in acid resistance, and therefore the silver ion readily flows away into an acidic solution and the microbicides have no durable antimicrobial effect. Furthermore, silver ion is unstable against exposure to heat and light and is immediately reduced to metallic silver to cause coloration. Thus, these microbicides lack long-term stability.
In order to enhance the stability of silver ion, an attempt has been made to support both silver ion and ammonium ion on zeolite by ion exchanging, but the problem of coloration has not yet been solved to practically acceptable level and fundamental solution has not yet been attained.
Furthermore, as other inorganic microbicides, those which comprise antimicrobial metals supported on active carbons having adsorbability have been proposed. However, in these microbicides soluble antimicrobial metal salts are merely physically adsorbed or deposited, and hence the antimicrobial metal ions rapidly dissolve away upon contact with water and the microbicides have no prolonged antimicrobial effect.
Recently, it has been proposed to use as a microbicide an antimicrobial compound comprising an antimicrobial metal ion supported on a specific zirconium phosphate such as Ag.sub.0.01 H.sub.0.95 Li.sub.0.04 Zr.sub.2 (PO.sub.4).sub.3. This microbicide is chemically and physically stable and is known as a material which has long-term antifungal and antibacterial activities (Japanese Patent Kokai No. 3-83905).
However, the process for preparing the antimicrobial compound proposed in Japanese Patent Kokai No. 3-83905 uses a phosphate prepared by a dry process and according to the dry process, the antimicrobial ion can be uniformly supported on the zirconium phosphate only after a mixture of starting material powders has been fired to obtain a lumpy zirconium phosphate and then this has been crushed and ground to fine powders. Therefore, this process has the problems that it is low in productivity; additionally it is difficult to obtain a phosphate compound having uniform and fine particle size.
Moreover, this antimicrobial compound tends to color slightly when used under severe conditions such as exposure to sunlight under high temperature and high humidity. Accordingly, there is a great need to inhibit even the slight coloration for uses in which coloration must be avoided as much as possible.
The object of the present invention is to provide a process for easily producing an antimicrobial compound having uniform and fine particle size and usable as a microbicide which undergoes substantially no coloration even under severe conditions such as exposure to sunlight or high temperature or contact with acidic solutions and which can exhibit antifungal, antibacterial and antialgal activities for a long period of time, without using crushing and grinding steps which are required for preparing the phosphates by a dry process.